Type soundness

We show a weak type soundness result for the abstract semantics
of Mach: for a well-typed Mach program, if a transition is taken
from a state where registers hold values of their static types,
registers in the final state hold values of their static types
as well. This is a subject reduction theorem for our type system.
It is used in the proof of implication from the abstract Mach
semantics to the concrete Mach semantics (file Machabstr2concr).